System and method for enhancing coal bed methane recovery

ABSTRACT

A system includes first and second wells. The first well has a first tube that extends from a first well head to a first end disposed within a coal seam. The second well is disposed at a distance from the first well and includes a second tube that extends from a second well head to a second end disposed within the coal seam. A pump is coupled to the first well and is configured to supply the first tube with pressurized fluid that includes nutrients for methanogenesis. At least a portion of the pressurized fluid introduced into the first tube of the first well is received within the second tube of the second well by way of the coal seam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 61/374,796, filed Aug. 18, 2010, the entirety of which isherein incorporated by reference.

FIELD OF DISCLOSURE

The disclosed systems and methods relate to the production of methanegas. More specifically, the disclosed systems and methods relate to theinjection of nutrients, which may include metabolic amendments, and/ormicroorganisms for microbially-enhanced coal bed natural gas (e.g.,methane or “coal bed methane”) recovery.

BACKGROUND

Many coal seams around the world either have produced or are capable ofproducing biogenic methane. Biogenic methane was created initiallythrough a process known as a methanogenesis, which is a naturallyoccurring process that has been in existence for millions of years.

Recently, laboratory studies have duplicated the methanogenesis processand have created new biogenic gas in relatively short time periods, insome instances, as few as twenty (20) days. After completion of theselaboratory studies, field pilot studies were initiated in an attempt toduplicate the findings in previous lab studies. Field pilot programshave replicated laboratory studies in that new biogenic methane wasproduced in several coal bed methane wells that prior to the study werecompletely void of gas. However, these field pilot programs have notresulted in a wide-distribution of the nutrients and/or microbes.

SUMMARY

In some embodiments, a system includes first and second wells. The firstwell has a first tube that extends from a first well head to a first enddisposed within a coal seam. The second well is disposed at a distancefrom the first well and includes a second tube that extends from asecond well head to a second end disposed within the coal seam. A pumpis coupled to the first well and is configured to supply the first tubewith pressurized fluid that includes nutrients for methanogenesis. Atleast a portion of the pressurized fluid introduced into the first tubeof the first well is received within the second tube of the second wellby way of the coal seam.

In some embodiments, a method includes injecting a fluid having a firstnutrient concentration into a coal seam through a first well under afirst pressure and extracting a second fluid having a second nutrientconcentration from the coal seam through a second well disposed apartfrom the first well. The second nutrient concentration is less than afirst nutrient concentration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of one example of a method of increasingmethanogenesis in a coal bed.

FIG. 2A 4 illustrates one example of a downhole arrangement of aninjection well including a flow directing nozzle.

FIG. 2B illustrates one example of a downhole arrangement of acirculating well.

FIG. 2C illustrates one example of a well-site arrangement used forinjecting nutrient-enriched fluid under pressure.

FIG. 2D illustrates one example of a well site arrangement for adaily/repetitive nutrient injection and circulation method.

FIG. 3A illustrates one example of a pattern of injection andcirculating wells in a site for producing coal bed methane.

FIG. 3B illustrates another example of a pattern of injection andcirculating wells in a site for producing coal bed methane.

FIG. 3C illustrates another example of a pattern of injection andcirculating wells in a site for producing coal bed methane.

DETAILED DESCRIPTION

The disclosed systems and methods advantageously enable distribution ofnutrients throughout a coal bed to provide a maximum exposure of themicroorganisms to coal pore space and surface area of coal beds. Theexposure of the microorganisms to the coal pore space and surface areais maximized by forcing nutrient rich water through the pore spaceitself thereby enabling the full potential of a methanogenesis processto convert the soluble or free carbon to methane. The system alsoadvantageously establishes routes for the resultant methane to flow andbe extracted.

For example, the system includes one or more pressurized pumps disposedadjacent to an injection well. The pumps inject nutrient-rich water intothe injection well. The water is circulated through the coal bed by theinjection pump and one or more circulation pumps disposed at a distancefrom the injection pump. The setup and operation of such systems may beperformed in accordance with an improved method.

FIG. 1 is a flow diagram of one example of a method 100 of setting upand operating an improved circulation system. As shown in FIG. 1, a testhole is drilled from the ground surface through the bottom of the coalseam at block 102. At block 104, the test hole is used to assesscharacteristics of the coal seam. For example, the structure of anoverburden layer can be analyzed and the depths of the overburden, thecoal seam, and the layers that separate multiple coal seams can bedetermined. Additionally, the porosity, elemental constitutions, andheat value may also be determined. Coal samples may be used to set upmicrocosms to determine the feasibility and potential of producingbiogenic natural gas in a laboratory. The depth of the coal, slope ofthe coal seam, and the structure of the coal may also be analyzed anddetermined at block 104. Formation/ground water may be collected atblock 104 to determine the chemical and biological characteristics andbe used in laboratory tests. The water depth, formation water rechargerate, and amount of water available may also be determined at block 104.

At block 106, an injection well is drilled in a site. The size and depthof the injection well may be based on the characteristics of the coal asdetermined at block 104. FIG. 2A is a cross-sectional view of oneexample of an injection well 200. As shown in FIG. 2A, injection well200 includes a well head 202 coupled to a casing 204 sized andconfigured to house a tubing 206. Casing 204 supports and protectstubing 206 from surrounding rock or earth 10 and extends from well head202 to coal seam 12. Tubing 206 is coupled to and extends from well head202 and nozzle 208, which is embedded within coal seam 12, and isconfigured to receive and transport fluid 14. Nozzle 208 may includesone or more apertures 210 configured to expel fluid in variousdirections.

Injection well 200 may be coupled to an injection pump 250 asillustrated in the embodiment shown in FIG. 2B. As shown in FIG. 2B,pump 250 may be disposed between well head 202 and a water/nutrientsource tank 260. A conduit 270 is coupled to tank 260 and to tubing 306such that nutrient-rich fluid 14 may be transferred from tank 260through injection well 200 into coal seam 12. In some embodiments, pump250 is configured to deliver between approximately 500 and 6,000 gallonsof nutrient-rich fluid into the coal seam 10 in conjunction with orfollowed by a volume of non-nutrient rich fluid between approximately5,000 and 15,000 gallons. One skilled in the art will understand thatthe other amounts of nutrient-rich water and non-nutrient rich water maybe adjusted based on the size of the coal seam 12, i.e., greater or lessthan the identified ranges. The clean water flush (i.e., non-nutrientrich water flush) pushes the initial nutrient-rich water out of thefractures in cleats and into the pore space of the coal. Upon thecompletion of injection, the injection well may be used as a recoveryand/or circulation well.

A second well, which may be a recovery and/or circulation well, isdrilled at a distance from the injection well at block 108. As will beunderstood by one skilled in the art, the distance at which theinjection well is positioned from the test well may be based on theinitial permeability assessment of the coal performed at block 104.

An example of such a recovery/circulation well 220 is illustrated inFIG. 2C. As shown in FIG. 4, recovery/circulation well 220 includes awell head 222 coupled to a casing 224. Casing 224 extends between wellhead 222 and coal seam 10 and is configured to receive and protecttubing 226. Tubing 226 is coupled to an intake nozzle 228, which isconfigured to receive fluid 14 from coal seam 12. Nozzle 228 may includeone or more apertures through which the fluid is received.

In some embodiments, such as the embodiment illustrated in FIG. 2D,injection well 200 and recovery/circulation well 220 are coupledtogether by a conduit 270 such that fluid 14 is recycled and reused. Asshown in FIG. 2D, well head 222 of recovery/circulation well 220 iscoupled to well head 202 of injection well 200 by conduit 270. Aninjection pump 250 is disposed along the length of conduit 270 and isconfigured to force pressurized fluid 14 into well head 202 and extractfluid from well head 222. The extracted fluid 14 received from well head222 may be passed through a nutrient injection system 280 along conduit270 to increase a concentration of nutrients for methanogenesis andother microbial pathways including, but not limited to, fermentation,facultative oxidation, and acetogenesis by the time it is injected intocoal seam 12 by injection well 200.

The nutrient injection may be implemented by gravimetric and/or lowpressure (e.g., approximately less than or equal to 50 psi). Injectionsystem 280 may include a mixing system comprising one or more tanks usedfor mixing nutrients with other chemical amendments or with a tracer.The one or more mixing tanks are filled with water from well 222 and/orfrom make-up water from another formation water source. The nutrientsand other chemical amendments or tracer is mixed in the one or moremixing tanks while being purged with an inert gas such as, for examplenitrogen or argon. Mixing is conducted by impellers, pumps, gasdiffusion, or any combination of methods as will be understood by oneskilled in the art. The mixture from the mixing tanks are injectedin-line with conduit 270 into well 202.

Both injection wells 200 and circulating wells 220 are capable ofproducing new biogenic gas generated from the circulation methodology.For example, each of the injection wells 200 and recovery/circulationwells 220 may be tied to a gathering system for transfer to a salesfacility.

Referring again to FIG. 1, a tracer fluid is injected into the injectionwell and removed (e.g., pumped out) from the second well at block 110.The tracer fluid injected through the injection well may have knowncharacteristics as well as be injected at a known rate. Examples of suchtracer fluid include, but are not limited to, sodium bromide andpotassium bromide. The tracer may pumped into the injection well inwater in which the concentration of the tracer is between approximately20-1,000 mg per liter of water. The tracer may be removed from thesecond well using a pump operating a second rate that may be differentfrom a first rate at which the injection pump pumps the tracer fluidinto the injection well.

At block 112, the fluid 14 removed from the second well is analyze todetermine the amount of tracer recovered such that the fluid connectionbetween the injection well and the second well may be determined. Forexample, if fifty percent or more of the injected tracer is recoveredfrom the second well, then it may be determined that a sufficient fluidconnection between the injection well and the second well has beenestablished. One skilled in the art will understand that other thresholdvalues may be used other than fifty percent.

The rates at which the tracer fluid is pumped into the injection welland pumped out of the test well may be measured to provide a real-timemeasurement of the permeability of the coal seam at block 114. Thereal-time permeability measurement of the coal may be used to adjust thepumping parameters of the injection pump and the extraction pump.

At block 116, one or more additional wells may be drilled at distancesfrom the injection well. As will be understood by one skilled in theart, the one or more additional wells may be drilled at distances basedon the real-time permeability of the coal over an area in which the coalis to be used to produce methane. The one or more wells may include oneor more injection wells 200 and/or one or more recovery/circulationwells 220. As described above, injection wells 200 may be coupled to atank 260 or to an output of a recovery/circulation well 220 through aconduit 270 and pump 250.

FIG. 3A illustrates one example of a site 300A in which a plurality ofwells are drilled to extract methane from coal. Site 300A may be dividedinto a number of subdivisions in which the number of subdivisions 302 isbased on the permeability of the coal. For example, site 300 may have anarea of approximately 40 acres and each subdivision 302 has an area ofapproximately 2.5 acres. In some embodiments, each of the subdivisions302 has an approximately equal area to form a grid, although one skilledin the art will understand that area 300 may be divided intosubdivisions 302 having differing areas and do not form a grid.

One or more wells 200, 220 may be disposed in each of the subdivisions302. For example, subdivisions 302-6, 302-7, 302-10, and 302-11 eachinclude an injection well 200 associated with a corresponding injectionpump 250. Each of the subdivisions 302 in which an injection pump 200and injection well 250 are not disposed, i.e., subdivisions 302-1:302-5,302-8, 302-9, and 302-12:302:16, may be configured with a respectiverecovery/circulation well 220 and corresponding pump 250.

In embodiments in which the well heads 202 of injection wells 200 arecoupled to the well heads 222 of recovery/circulation wells 220, such asthe embodiment illustrated in FIG. 2B, conduits 270 may extend from onesubdivision 302 to an adjacent subdivision. For example, injection well200-1 in subdivision 302-6 may received fluid from recovery/circulationwell 220-2 in subdivision 302-2 as identified by the arrow 304.Similarly, injection well 200-2 in subdivision 302-7 may receive fluidfrom recovery/circulation well 220-6 in subdivision 302-8 as identifiedby arrow 304 extending between the two wells.

One skilled in the art will understand that wells 200, 220, and pumps250 may be configured in other patterns with respect to subdivisions.For example, FIG. 3B illustrates another embodiment of a site 300Bdivided into a plurality of subdivisions 302, but each subdivision 302does not include a respective well 200, 220. As shown in FIG. 3B,subdivisions 302-1, 302-3, 302-6, 302-8, 302-9, 302-11, 302-13, and302-16 do not include an injection well 200 nor a recovery/circulationwell 220. Each pair of injection wells 200 and recovery/circulation well220 is disposed in non-vertically and horizontally aligned subdivisions.For example, an injection well 200-1 is disposed in subdivision 302-4and is fluid communication with recovery/circulation well 220-1, whichis disposed in subdivision 220-1, and injection well 200-2 is disposedin subdivision 302-7 and is coupled to recovery/circulation well 220-2disposed in subdivision 302-4. Injection wells 200-3, 200-4 disposed insubdivisions 302-10, 302-12 are respectively coupled torecovery/circulation wells 222-3, 220-4 disposed in subdivisions 301-13,302-16.

FIG. 3C illustrates another embodiment in which a single injection well200 disposed at an approximate center of site 300C and coupled to one ormore recovery/circulation wells 220. As shown in FIG. 3C,recovery/circulation wells 220-1, 220-2, 220-3, and 220-4 are disposedin the corner subdivisions 302-1, 302-4, 302-13, and 302-16 of site300C. Circulation wells 220 are each coupled to injection well 200 viainjection pump 250.

Referring again to FIG. 1, fluid is injected into a coal seam atinjection wells 200 at block 118. Injection wells 200 and pumps 250 areconfigured to inject nutrient-rich fluid, e.g., water, into coal seams14 under pressure. In some embodiments, the nutrient-rich fluid isinjected into coal seams 14 under a pressure of up to and including 100psi. One skilled in the art will understand that less pressure orgreater pressure may be used to inject nutrient-rich fluid into coalseams 14 via injection wells 200. The amount of nutrient-rich fluidinjected into a coal seam may also vary based on an area of the site andsize of the coal seam. For example, approximately 500 and 6,000 gallonsof nutrient-rich fluid may be injected at an injection well 200 in a 40acre site.

At block 120, a non-nutrient enriched fluid may be injected into thecoal seam via the injection well(s) 200. For example, approximately5,000 to 15,000 gallons of non-nutrient enriched fluid may be injectedinto the coal seam 14 through injection well(s) 200 in a 40 acre site.One skilled in the art will understand that other amounts ofnon-nutrient enhanced fluid may be injected based on the size of thecoal seam, i.e., greater or less than the identified range. Thenon-nutrient enriched fluid flush pushes the initial nutrient-rich fluidout of the fractures in cleats and into the pore space of the coal.

At block 122, recovery/circulating wells 220 are turned to move thenutrients away from the injection wells 200 to spread the nutrientsthroughout the entire coal seam 14. In some embodiments, circulatingpumps 220 are configured to move fluid in a range from 5 gallons perminute to 200 gallons per minute depending on the size of the site andthe number of injection wells 200 and/or recovery/circulation wells 220disposed in the site. One skilled in the art will understand thatcirculating pumps 220 may be configured to move fluid with other flowrates.

As described above, the nutrient-depleted fluid extracted fromrecovery/circulating wells 220 may pass through a nutrient injectionsystem (280 in FIG. 2D) and then pumped into injection wells 200 byinjection pumps 250. In some embodiments, the process is repeated on adaily basis running 24 hours per day until such time as the optimalreservoir saturation is achieved. The saturation point will be reportedin field monitoring systems and can be read via a supervisory controland data acquisition (“SCADA”) system in the field or at other offices.In some embodiments, soluble carbon sources, such as carbon dioxide, maybe injected by an injection well 200 to increase methanogenesis. Thedisclosed systems and methods advantageously increase methanogenesis byincreasing the amount of nutrients in the coal seam.

Although the systems and methods have been described in terms ofexemplary embodiments, they are not limited thereto. Rather, theappended claims should be construed broadly, to include other variantsand embodiments of the systems and methods, which may be made by thoseskilled in the art without departing from the scope and range ofequivalents of the systems and methods.

What is claimed is:
 1. A system, comprising: a first well including afirst tube that extends from a first well head to a first end disposedwithin a coal seam; a second well disposed at a distance from the firstwell, the second well including a second tube that extends from a secondwell head to a second end disposed within the coal seam; and a pumpcoupled to the first well and configured to supply the first tube withpressurized fluid that includes nutrients for methanogenesis, wherein atleast a portion of the pressurized fluid introduced into the first tubeof the first well is received within the second tube of the second wellby way of the coal seam.
 2. The system of claim 1, wherein the firstwell includes a nozzle disposed at the first end configured with aplurality of aperture for directing the pressurized fluid into the coalseam.
 3. The system of claim 1, further comprising: a third welldisposed at a distance from the first and second wells, the third wellincluding a third tube that extends from a third well head to a thirdend disposed within the coal seam, wherein at least a portion of thepressurized fluid introduced into the first tube of the first well isreceived within the second tube of the second well by way of the coalseam.
 4. The system of claim 1, further comprising: a third welldisposed at a distance from the first and second wells, the third wellincluding a third tube that extends from a third well head to a thirdend disposed within the coal seam; and a second pump coupled to thethird well and configured to supply the third tube with pressurizedfluid that includes nutrients for methanogenesis, wherein at least aportion of the pressurized fluid introduced into the first and secondtubes of the first and second wells is received within the second tubeof the second well by way of the coal seam.
 5. The system of claim 1,wherein the fluid is provided to the pump from a tank.
 6. The system ofclaim 1, wherein the first tube is connected to the second tube by aconduit that extends between the first well head and the second wellhead.
 7. The system of claim 6, wherein the pump is disposed along thelength of the conduit.
 8. The system of claim 7, wherein a nutrientinjection system is disposed along the length of the conduit.
 9. Amethod, comprising: injecting a fluid having a first nutrientconcentration into a coal seam through a first well under a firstpressure; and extracting a second fluid having a second nutrientconcentration from the coal seam through a second well disposed apartfrom the first well, wherein the second nutrient concentration is lessthan a first nutrient concentration.
 10. The method of claim 9, furthercomprising: drilling a test well into the coal seam; assessing at leastone characteristic of the coal seam using the test well; drilling thefirst well; and drilling the second well at a distance from the firstwell based on the at least one characteristic.
 11. The method of claim10, further comprising: injecting a tracer into the first well; andextracting at least a portion of the tracer from the second well. 12.The method of claim 11, further comprising obtaining a measurement ofpermeability of the coal seam based on a rate at which the tracer isinjected to the first well and the portion of the tracer is extractedfrom the second well.
 13. The method of claim 9, further comprisingextracting a third fluid having a third nutrient concentration from thecoal seam through a third well disposed apart from the first and secondwell.
 14. The method of claim 13, wherein the second and third nutrientconcentrations are less than the first nutrient concentration.
 15. Themethod of claim 9, further comprising: increasing the second nutrientconcentration of the second fluid to provide the first fluid having thefirst concentration at a nutrient injection system coupled to the secondwell; and supplying the first fluid to the first well by way of aconduit between the nutrient injection system and the first well. 16.The method of claim 9, further comprising: injecting a third fluidhaving a third nutrient concentration into the first well after thefirst fluid has been injected into the first well, wherein the thirdnutrient concentration is less than the first and second nutrientconcentrations.
 17. The method of claim 16, wherein the third fluidforces the first fluid into spaces of the coal seam and combined withthe first fluid to create the second fluid having the secondconcentration.
 18. The method of claim 9, wherein the nutrients are forproducing methanogenesis.
 19. The method of claim 9, wherein the firstpressure is up to and including approximately 100 pounds per squareinch.